1. Field of the Invention
The present invention relates to a brake apparatus provided with a rotary type pump in particular, having an internal gear pump such as a trochoid pump for sucking and discharging fluid and a method of manufacturing the same.
2. Related Arts
An internal gear rotary pump such as a trochid pump and the like is constituted by an inner rotor having outer teeth portion at an outer periphery thereof, an outer rotor having inner teeth portion at an inner periphery thereof, and a casing for containing the outer and inner rotors. The inner rotor and the outer rotor are arranged in the casing in a state where the inner and outer teeth portions are in mesh with each other and a plurality of gaps are formed by the respective teeth portions.
When a line running on respective central axes of the outer and inner rotors is defined as a centerline of the pump, an intake port and a discharge port, which communicate with the plurality of gaps, respectively, are formed on both sides of the centerline. When the pump is driven, the inner rotor rotates with the central axis as a drive axis. In accordance therewith, the outer rotor also rotates in the same direction as the inner rotor by mesh between the outer and inner teeth portions. In this case, each of the gaps changes from a large volume to a small volume and vice versa during a time period in which the outer rotor and the inner rotor make one turn. Due to that volume change, fluid is sucked from the intake port and discharged to the discharge port.
Conventionally, to cope with wear and tear of the outer and inner rotors in the rotary pump mentioned above, the rotors are made of sintered material, the rotors made of sintered material are processed by carburising, the rotor are made of bearing steel or the rotors made of bearing steel are processed by quenching and tempering. However, when the rotary pump is used in the brake apparatus, processing the rotors through the carburising treatment mentioned above is not sufficient to secure high wear resistance required for the rotors since lubricity of brake fluid is low so that very high pressure occur on teeth faces of the rotors that are meshed with each other (in particular, at portions where the gap has a maximum volume). Further, the rotors may be made of tool steel such as SKH or SKD, which has higher wear resistance.
However, the tool steel is expensive and has poor forgeability. Furthermore, the rotors made of tool steel containing higher carbide content for enhancing the wear resistance is likely to crack at the portions of the carbide so that the rotors may not have sufficient strength. Moreover, the rotors made of conventional sintered material can""t have sufficient fatigue strength because of voids.
Therefore, it is an object of the present invention to provide a brake apparatus having a rotary type pump in which outer and inner rotors has higher wear resistance at the teeth faces where the rotors are meshed with each other.
To achieve the object mentioned above, the brake apparatus has a rotary pump comprising first and second rotors made of bearing steel each having teeth portions at least at one of inner and outer peripheries thereof, and a plurality of gaps formed between the teeth portions of the first and second rotors which are in mesh with each other so that brake fluid may be sucked into and discharged from the gaps according to rotations of the first and second rotors. With the rotary pump, the first and second rotors have circumferential surfaces of the teeth portions which are respectively processed by at least one of nitriding hardening and carbo-nitriding hardening.
Further, it is preferable that the rotary pump has a casing having intake and discharge ports, outer and inner rotors having inner and outer teeth portions at inner and outer peripheries thereof, respectively, so that the inner and outer rotors are housed in the casing so as to constitute a plurality of gaps formed between the inner and outer teeth portions which are in mesh with each other and the gaps are communicated with the intake and discharge ports, and a drive shaft for driving the inner rotor in the casing so that brake fluid may be sucked from the intake port and, via the gaps, discharged from the discharge port according to a rotation of the drive shaft.
With the rotary pump mentioned above, the outer and inner rotors are made of bearing steel and have circumferential surfaces of the inner and outer teeth portions that are respectively processed by at least one of nitriding hardening and carbo-nitriding hardening.
Furthermore, each of the circumferential surfaces of the teeth portions is provided, preferably, with nitrided layer having 15 to 40 percents retained austenite.
Moreover, in a brake apparatus having a main conduit communicating brake fluid of a master cylinder to a wheel cylinder and an auxiliary conduit bypassing the main conduit, the rotary pump may be disposed in the auxiliary conduit for sucking brake fluid from the auxiliary conduit on a side of the master cylinder and discharging the same to the auxiliary conduit on a side of the wheel cylinder so that brake fluid pressure of the wheel cylinder may be more increased than that of the master cylinder.
Another object of the present invention is to provide a method of manufacturing outer and inner rotors of the rotary pump in which outer and inner rotors has higher wear resistance at the teeth faces where the rotors are meshed with each other and has mirror finished surfaces at side surfaces of the rotors so as to limit brake fluid leakage between the side surfaces of the rotor and side plates of a casing.
To achieve the object mentioned above, in the rotary pump having inner and outer rotors provided with outer and inner teeth portions, respectively, which are contained in a center bore of a center plate so as to make the outer and inner teeth portions in mesh with each other and are put between first and second side plates, the method of manufacturing the outer and inner rotors comprises steps of, at first, forming the outer and inner teeth portions at inner and outer peripheries of pipe members made of bearing steel, respectively, then, executing at least one of nitriding and carbo-nitriding on the respective pipe member, then, cutting the respective pipe member into a small pieces to constitute a plurality of the respective outer and inner rotors, next quenching and tempering the outer and inner rotors, and, finally, executing mirror-like finishing on cutting faces of the outer and inner rotors.
With the steps mentioned above, mirror-like finished surfaces of the outer and inner rotors may face the first and second side plates, while teeth faces of the teeth portions in mesh are covered by nitrided layer containing 15 to 40 percent retained austenite.